why do we get zapped by door knobs? they're not grounded? well maybe they are when the door is closed...
that which is so special about earth is that it is a lower potential than certain things, me for example.What's so special about the earth?
The reason you get a shock to earth is that it is large enough that the small amount of charge flowing from you doesn't change it's potential (voltage) so the charge can keep flowing.
Simimairly with a door knob, it's a relatively large isolated metal object so can contain a large amount of charge - so when you touch it there is a large potential difference which doesn't decrease, allowing all the charge to keep flowing from you.
It's a bit like touching a bath of hot water rather than a tiny spark. The spark is hotter but soon cools, the water can keep putting heat into you for longer.